Protein aggregation and misfolding diseases, or proteopathies, constitute a broad range of debilitating diseases such as Alzheimer's disease, Parkinson's disease, cystic fibrosis, prion disease, type 2 diabetes, amyloidosis, and amyotrophic lateral sclerosis (ALS). Mutations or environmental stresses can cause proteins to assume abnormal structures (i.e., misfolding) that result in either the loss of the protein's original function or in the gain of a new, noxious function. At the molecular level, the progression of a proteopathy is frequently characterized by the sequential structural transformation of the protein involved that eventually results in protein aggregation, but the identity of the intermediate species responsible for the protein's toxicity is unknown in most cases.
ALS proteopathy is a rapidly progressing neurodegenerative disease with a varied etiology characterized by the gradual degeneration and death of motor neurons. Currently, there are no effective therapeutic agents for ALS, and there is only one FDA-approved drug for its treatment, riluzole, which only delays the need for a ventilator and may extend life by up to three months.
Approximately 20% of familial ALS cases result from mutations in the gene that encodes the enzyme copper-zinc superoxide dismutase 1 (SOD1), a ubiquitous 32 kDa homodimeric protein critical to cellular defense against reactive oxygen species. To date, over 150 mutations in SOD1 have been identified and described as causing ALS in a dominant fashion (FALS mutations), but their distribution throughout the SOD1 sequence exhibits no apparent pattern. In addition, it has been suggested that a significant fraction of sporadic ALS cases is caused by the exposure of wild type SOD1 (SOD1WT) to cellular stress, which may be cell- or tissue-specific. Phenotypically, SOD1-related ALS is universally manifested by SOD1 misfolding and aggregation, and the pathogenesis is attributed to novel noxious function(s) acquired by SOD1 upon misfolding. Downstream physiological effects of gain-of-function SOD1 toxicity include, among others, impaired mitochondrial metabolism, axonal degeneration, axonal transport failure, excitotoxicity, proteasomal disruption, and endoplasmic reticulum stress. The nature of the toxic SOD1 species and the mechanism of their toxicity remain obscure.
Disease-causing mutations may induce SOD1 misfolding by promoting SOD1 demetallation (i.e., formation of apo-SOD1) and the reduction of the stabilizing intra-subunit disulfide bond, thereby reducing the stability of SOD1 monomers or destabilizing the dimer interface. Mutations could also interfere with SOD1 binding to the copper chaperone for SOD1 (CCS), which normally recognizes newly synthesized SOD1 and activates it by facilitating the insertion of catalytic copper and the oxidation of the disulfide bond. With its progressively decreased global stability, misfolded SOD1 eventually precipitates to form amyloid-like aggregates. Soluble monomers and low-molecular weight oligomers formed along the aggregation pathway are particularly attractive candidate noxious species.
The acquired ability of misfolded SOD1 to form aberrant interactions with a variety of cellular proteins and interfere with their normal functions constitutes a potential pathogenic mechanism of SOD1-related ALS. Unlike SOD1WT, misfolded SOD1 mutants (SODMUT) interact with proteins such as HoxB2, a homeodomain-containing transcription factor, cytosolic malate dehydrogenase (MDH1), the voltage-dependent anion channel (VDAC1), Bcl-2, brain calcineurin, the axonal dynein complex, and derlin-1, among others. This ability to interact with structurally diverse proteins is a feature of proteins whose surfaces are characterized by the presence of conformationally adaptive interaction hot spot(s). Indeed, the ‘gain-of-interaction’ of misfolded SOD1 may indicate that upon misfolding, certain elements of the SOD1 surface acquire functional characteristics typical of energetic hot spots. Due to the high abundance of SOD1 (˜1% of total human proteins) and its ubiquity (extracellular, cytoplasmic, nuclear, intra-mitochondrial), such a ‘hot spot on the loose’ scenario may inflict substantial cell damage by perturbing proper PPI homeostasis. Selective damage to motor neurons in ALS may be caused by misfolded SOD1 interfering with the activity of protein(s) predominantly expressed in these cells or playing a vital role in cell homeostasis.
There is an unmet need for novel therapies for the treatment of ALS. Inhibition of aggregation of misfolded SOD1 may be valuable for the treatment and prevention of ALS.